Function pick 

pub fn pick<P: Selectable>(
    scene: &dyn Scene,
    reader: &Reader<'_>,
    pointer: &Pointer,
) -> Option<(P, Vector3)>

What a click with pointer on what the editor draws, scene, selects as a P in the part reader reads — for a step that picks on the canvas itself rather than through a reference field — and where it hit what is drawn.